Split variables from main font awesome less file

When you work with less and use font awesome, the current way of doing things is to download font awesome, change the font path variable in the main file and then include it. This works fine until you want to upgrade to a new release, when you will have to remember to patch the original source file with the fonts folder variable. This can be problematic, especially if you are using a package manager like bower to manage your dependencies automatically.

This pull request moves the font variable to a variables.less file, and adds a new less file, font.less, that can be included in custom projects.

The font-awesome.less file works exactly as it did before.

This is not a problem in the sass version of the project, since sass has the !default feature.

The new workflow when including font awesome in your less project would now be:

    @FontAwesomePath:   "path-to-font";
    @import "font.less";